Reduced Reperfusion–Induced Ins(1,4,5)P 3 Generation and Arrhythmias in Hearts Expressing Constitutively Active α 1B -Adrenergic Receptors

Abstract

Abstract —Reperfusion of globally ischemic rat hearts causes the generation of inositol(1,4,5) tris phosphate [Ins(1,4,5)P 3 ] and the initiation of arrhythmias. These responses are mediated by α 1 -adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic conditions, hearts from transgenic animals expressing constitutively active α 1B -ARs in heart (α 1B -constitutively active mutant [CAM]) showed higher [ 3 H] inositol phosphate responses to norepinephrine (2.3-fold) than hearts from nontransgenic animals (α 1B -WT) (1.6-fold). α 1B -WT hearts responded to 2 minutes of reperfusion after 20 minutes of global ischemia by generation of Ins(1,4,5)P 3 (5301±1310 to 11 413±1597 CPM/g tissue; mean±SEM; n=6; P <0.01 in [ 3 H] labeling studies and 3.8±0.2 to 6.3±0.6 nmol/g by mass analysis, n=6; P <0.05). In contrast to findings in normoxia, hearts from α 1B -CAM animals showed no Ins(1,4,5)P 3 response in early reperfusion. In parallel studies, α 1B -WT hearts developed ventricular tachycardia and ventricular premature beats (VPB) during 5 minutes of reperfusion after 20 minutes of ischemia. The incidence of these arrhythmias was reduced in the α 1B -CAM hearts (95% to 62% for VPB and 47% to 12% for ventricular tachycardia; both P <0.05). The resistance of the α 1B -CAM hearts was not due to α 1B -AR–mediated preconditioning, as the Ins(1,4,5)P 3 response to thrombin receptor activation during reperfusion was not different between the 2 groups. To investigate the possibility of reduced α 1A -receptor activity in the α 1B -CAM hearts, expression of the mRNA for α 1A - and α 1B -receptors was measured. α 1B -WT hearts contained mRNA for both receptor subtypes, but the levels of α 1B -receptor mRNA were 5-fold higher than α 1A -receptor mRNA. α 1B -CAM hearts contained very high levels of α 1B -receptor mRNA (26-fold increase), but the expression of mRNA for the α 1A -receptors (0.141±0.035 amol/μg RNA; mean±SEM; n=6) was reduced by 50% relative to α 1B -WT controls (0.276±0.046 amol/μg RNA; n=6; P <0.01). The reduction in arrhythmogenic and Ins(1,4,5)P 3 responses in α 1B -CAM hearts provides evidence that these response are not mediated by α 1B -receptors.